The present invention is directed toward an instrument for locating the apical foramen in the tooth of a dental patient and, more particularly, toward such a device which measures and displays the distance from a point within the root canal of a tooth to the apex and which then accurately displays that distance both visually and audibly.
The apical foramen or apex is that anatomical point where the nerves enter a tooth. The passageway that these nerves follow once in the tooth is known as the root canal. When a tooth dies, there are two options: extraction or root canal therapy. Root canal therapy is a procedure wherein all of the dead tissue known as the pulp is removed from the tooth, leaving the bone which is functional for chewing even though the tooth is dead.
The location of the apical foramen is important to the root canal therapy procedure because the success of it depends on fully removing all dead tissue while not going past the apex which could cause an abscess by pushing the dead tissue into the tissue of the jaw. Originally, this point was located by successive X-rays. That is, an instrument was advanced into the canal a small amount, an X-ray was taken to find the location and this procedure was repeated until completed. It was well recognized, however, that this X-ray technique was inadequate and undesirable in that it resulted in too much exposure to X-rays and as a result of the fact that the nerve very often does not exit at the tip of the root. Depending on the angle of the X-ray, the reamer may, in fact, be very far past the apex when it appears to be at the tip of the root.
In the early 1960's, electronic devices were proposed for locating the apex by measuring the electrical resistance between an anode inserted into the canal and a cathode placed on a part of the human body such as the buccal mucous membrane.
It was empirically found that the conductance from the apical foramen to the lip was the same as the conductance from the gum line at the tooth (a point known as the sulcus) to the lip. Therefore, by measuring the conductance from the sulcus to the lip prior to root canal therapy, and looking for that same conductance measurement from the metallic reamer to the lip during the root canal procedure, one could electronically locate the apical foramen.
Prior U.S. Pat. No. 4,526,179 more specifically describes the history of electronic apical foramen locating apparatuses and discusses the deficiencies of each of the mentioned prior art devices. Rather than repeat that discussion at length, the text of that former patent is incorporated herein by reference.
The device shown in U.S. Pat. No. 4,526,179 was intended to overcome the deficiencies of the prior art discussed therein and provided a digital apical foramen locating apparatus which utilized the known equality in the conductances from the apex to the lip and from the lip to the sulcus as discussed above to numerically display the distance from a reamer to the apex. While this device has met with much success, it has been found that the numerical display does not adequately satisfy all doctors utilizing the device. Furthermore, this prior device, although calibrated at the factory, has been known to be subject to some drift due to aging of the components and ambient conditions.